The invention relates to a cathode ray tube fitted with at least one cathode comprising a cathode carrier with a cathode base of a first cathode metal and a cathode coating of an electron-emitting material containing a second cathode metal and at least one alkaline earth oxide selected among the group consisting of the oxides of calcium, strontium and barium.
A cathode ray tube is composed of four functional groups:                electron beam generation in the electron gun,        beam focusing using electrical or magnetic lenses,        beam deflection to generate a raster, and        luminescent screen or display screen.        
The functional group relating to electron beam generation comprises an electron-emitting cathode, which generates the electron current in the cathode ray tube and which is enclosed by a control grid, for example a Wehnelt cylinder having an apertured diaphragm on the front side.
An electron-emitting cathode for a cathode ray tube generally is a punctiform, heatable oxide cathode with an electron-emitting, oxide-containing cathode coating. If an oxide cathode is heated, then electrons are evaporated from the electron-emitting coating into the surrounding vacuum.
The quantity of electrons that can be emitted from the cathode coating depends on the work function of the electron-emitting material. Nickel, which is customarily used for the cathode base, has itself a comparatively high work function. For this reason, the metal of the cathode base is customarily coated with another material, which mainly serves to improve the electron-emitting properties of the cathode base. A characteristic feature of the electron-emitting coating materials of oxide cathodes is that they comprise an alkaline earth metal in the form of the alkaline earth metal oxide.
To manufacture an oxide cathode, a suitably shaped sheet of a nickel alloy is coated, for example, with the carbonates of the alkaline earth metals in a binder preparation. During evacuating and baking out the cathode ray tube, the carbonates are converted to the oxides at temperatures of approximately 1000° C. After this burn-off of the cathode, said cathode already supplies a noticeable emission current which, however, is still unstable. Next, an activation process is carried out. This activation process causes the originally nonconducting ionic lattice of the alkaline earth oxides to be converted to an electronic semiconductor in that donor-type impurities are incorporated in the crystal lattice of the oxides. These impurities essentially consist of elementary alkaline earth metal, for example calcium, strontium or barium. The electron emission of the oxide cathodes is based on the impurity mechanism. Said activation process serves to provide a sufficiently large quantity of excess, elementary alkaline earth metal, which enables the oxides in the electron-emitting coating to supply the maximum emission current at a prescribed heating capacity. A substantial contribution to the activation process is made by the reduction of barium oxide to elementary barium by alloy constituents (“activators”) of the nickel from the cathode base.
For the function and the service life of an oxide cathode it is important that elementary alkaline earth metal is continuously dispensed. The reason for this being that the cathode coating continuously loses alkaline earth metal during the service life of the cathode. The cathode material partly evaporates slowly and is partly sputtered off by the ion current in the cathode ray tube.
However, initially the elementary alkaline earth metal is continuously dispensed. Said dispensation of elementary alkaline earth metal by reduction of the alkaline earth oxide at the cathode metal or activator metal stops, however, when a thin, yet high-impedance interface of alkaline earth silicate or alkaline earth aluminate forms between the cathode base and the emitting oxide in the course of time. The service life is also influenced by the fact that the amount of activator metal in the nickel alloy of the cathode base becomes depleted in the course of time.
JP 11204019 A discloses an oxide cathode with improved donor density and a longer service life, which comprises a cup of a nickel alloy, which is filled with a clew of a nickel alloy and with an alkaline earth carbonate mixture.
It is an object of the invention to provide a cathode ray tube, the beam current of which is uniform and remains constant for a long period of time, while said cathode ray tube can be reproducibly manufactured.
In accordance with the invention, this object is achieved by a cathode ray tube fitted with at least one oxide cathode, which comprises a cathode carrier having a cathode base of a first cathode metal with a covering layer composed of ultrafine metal particles that contain nickel, and which further comprises a cathode coating of an electron-emitting material containing a particle-particle composite material of oxide particles and metal particles, which oxide particles comprise an oxide selected among the oxides of scandium, yttrium and the lanthanoids cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and an alkaline earth oxide selected among the group consisting of the oxides of calcium, strontium and barium, and the metal particles contain a second cathode metal selected from the group consisting of Ni, Co, Ir, Pd, Rh and Pt.
A cathode ray tube comprising such an oxide cathode has a constant beam current for a long period of time, which can be attributed to the fact that the homogeneous distribution of the reductive cathode metal and the activator metal in the material of the electron-emitting cathode coating causes the growth of high-impedance intermediate layers to be locally distributed and reduced. Elemental barium can be dispensed for a longer period of time. The effect of the coating layer composed of ultrafine metal particles containing nickel is very advantageous. Said coating layer forms a disintegrated boundary between the cathode base and the cathode coating. As a result the formation of a high-impedance deactivated interlayer between the cathode base and the cathode coating becomes discontinuous and the resistance of the high-impedance interlayer is reduced. Local activator dispensation and activator diffusion are enhanced.
As barium is dispensed continuously, depletion of the electron emission, as known from the oxide cathodes according to the prior art, is precluded. Substantially higher beam current densities can be obtained without adversely affecting the service life of the cathode. This can also be used to draw the necessary electron beam currents from smaller cathode regions. The spot size of the cathode spot determines the beam focusing quality on the display screen. The picture definition is increased throughout the screen. As, in addition, aging of the cathodes is a very slow process, picture brightness and picture definition can be maintained at a high level throughout the service life of the tube.
For the first cathode metal use is preferably made of a metal selected among the group consisting of Ni, Co, Ir, Re, Pd, Rh and Pt.
It is particularly preferred that the first cathode metal contains an alloy of a metal selected among the group consisting of Ni, Co, Ir, Re, Pd, Rh, Pt and an activator metal selected among the group consisting of Mg, Mn, Fe, Si, W, Mo, Cr, Ti, Hf, Zr, Al.
In accordance with a preferred embodiment, the covering layer additionally comprises an activator metal selected among the group consisting of Mg, Mn, Fe, Si, W, Mo, Cr, Ti, Hf, Zr, Al. By virtue thereof, the sensitivity to “poisioning” by residual gases in the crt-vacuum is reduced.
It is particularly preferred that the metal particles comprise a slow activator selected among the group consisting of Al, Mo, Ti and Si. The slow activators are preferably added in a quantity ranging from 1 to 4% by weight.
It may alternatively be preferred that the metal particles in the electron-emitting material comprise an alloy of a second cathode metal selected among the group consisting of Ni, Co, Ir, Re, Pd, Rh, Pt and an activator metal selected among the group consisting of Mg, Mn, Fe, Si, W, Mo, Cr, Ti, Hf, Zr, Al.
The oxide particles may comprise oxide particles of an alkaline earth oxide selected among the group of oxides consisting of calcium, strontium and barium, which is doped with an oxide selected among the oxides of scandium, yttrium and the lanthanoids cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.
In accordance with a particularly preferred embodiment, the oxide particles comprise oxide particles of an alkaline earth oxide selected among the group of oxides consisting of calcium, strontium and barium, which is doped with one of the oxides of yttrium. It has surprisingly been found that yttrium oxide accelerates the sintering of the oxides in the manufacturing process.
In accordance with a further embodiment of the invention, the oxide particles comprise oxide particles of an oxide selected among the oxides of scandium, yttrium and the lanthanoids cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and oxide particles of an alkaline earth oxide selected among the group of oxides consisting of calcium, strontium and barium.
The electron-emitting material may contain metal particles in a quantity ranging from 1 to 5% by weight.
It is particularly preferred that the electron-emitting material contains nickel particles in a quantity of 2.5% by weight.
Particularly advantageous effects of the invention in relation to the prior art are achieved if the metal particles are shaped so as to be ellipsoidal or spherical. By virtue thereof, diffusion of the activator metals takes place in a more controlled manner and a more uniform barium emission, as regards time and place, is achieved. Oxide cathodes having a higher direct current carrying capacity and longer service life are obtained.
If the metal particles are needle-shaped, this may help to keep the diffusion of the activator metals constant throughout the service life of the oxide cathode.
The average particle diameter of the metal particles preferably ranges from 0.2 to 5.0 μm.
It may also be preferred that the metal particles are embedded in the particle-particle composite so as to be oriented, particularly that the metal particles are embedded in the particle-particle composite so as to extend vertically to the surface of the cathode base.
Alternatively, the metal particles are embedded in the particle-particle composite with a concentration gradient.
The invention also relates to an oxide cathode, which comprises a cathode carrier having a cathode base of a first cathode metal with a covering layer composed of ultrafine metal particles that contain nickel, and having a cathode coating of an electron-emitting material containing a particle-particle composite material of oxide particles and metal particles, which oxide particles comprise an oxide selected among the oxides of scandium, yttrium and the lanthanoids cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and an alkaline earth oxide selected among the group consisting of the oxides of calcium, strontium and barium, and the metal particles contain a second cathode metal selected from the group consisting of Ni, Co, Ir, Pd, Rh and Pt.
These and other aspects of the invention will be apparent from and elucidated with reference to one embodiment described hereinafter.